Some or all of the following abbreviations can be found in this application and/or in documents referenced by this application.    3GPP Third Generation Partnership Project    AAA Authentication, Authorization, and Accounting    AKA Authentication and Key Agreement    AuC Authorization Center    AUTN Authentication Token    CK Cipher Key    EAP Extensible Authentication Protocol    GETCERT Internet draft “Client Certificate and Key Retrieval for IKE”    GMM GPRS Mobility Management    GPRS General Packet Radio Service    GSM Global System for Mobile Communication    HMAC Message Authentication Using Cryptographic Hash Functions    HTTP Hyper Text Transfer Protocol    HSS Home Subscriber Server    IETF Internet Engineering Task Force    IK Integrity Key    IKE Internet Key Exchange    IMS IP Multimedia Subsystem    IMSI International Mobile Subscriber Identity    IP Internet Protocol    IPsec Internet Protocol Security    ISAKMP Internet Security Association and Key Management Protocol    ISIM IP Multimedia Services Module (may be implemented using the USIM)    MITM Man-in-the-Middle, a type of attack on a tunneled authentication protocol    NAS Network Authentication Server    OWLAN Operator Wireless Local Area Network    PEAP Protected EAP Protocol    PIC A Pre-IKE Credential Provisioning Protocol    PPP Point-to-Point Protocol    pppext “Point-to-Point protocol extensions” working group    RADIUS Remote Authentication Dial In User Service    RAND Random Challenge, generated by the AuC using SQN    RAS Remote Authentication Server    RES Authentication Response generated by ISIM    SIM Subscriber Identity Module    SQN Sequence Number    TLS Transport Layer Security    TTLS Tunneled TLS    UMTS Universal Mobile Telecommunications System    USIM Universal Subscriber Identity Module    WLAN Wireless Local Area Network
Authentication protocols are used in communication networks to enable one entity in the network to satisfy another entity of its capacity to be provided with a service, for example, the undertaking of a data transaction or the granting of access to a resource. As one example, if a user's mobile phone is to access an e-mail account on a remote server then the server may wish to authenticate the mobile phone so that it can be sure that it is making the contents of the account available to the correct entity. Similarly, the user of the mobile phone may desire that the mobile phone authenticate the server so that the user can be sure that data provided to the server is not being intercepted by a third party that is impersonating the server.
A number of authentication protocols have been developed. Examples include the EAP procedure, described in RFC 2284, that provides a standard mechanism for support of multiple authentication methods. Through the use of EAP, support for a number of authentication schemes maybe added, including smart cards, Kerberos, Public Key, One Time Passwords, and others. Specific EAP/SIM and EAP/AKA methods have been defined for the purposes of the OWLAN system, which makes use of the GSM or UMTS authentication methods for the authentication of WLAN access and derivation of link keys for the protection of the WLAN link.
EAP is a general authentication protocol, designed to allow end-points to use multiple forms of authentication. EAP does not require the server (typically a PPP or IEEE 802 end-point) to authenticate the client itself, rather it allows the server to proxy authentication messages to a back-end authentication server, and to inspect the packets to determine if the authentication was successful.
One of the goals of EAP is to enable the development of new authentication methods without requiring the deployment of new code on the local authentication server. As a result, the local server acts as a “passthrough”, and need not understand specific EAP methods.
Since its deployment, a number of weaknesses in EAP have become apparent. These include a lack of protection of the user identity or the EAP negotiation, as well as no standardized mechanism for implementing a key exchange. For example, EAP/SIM and EAP/AKA solve these problems by making use of the specific features of SIM and USIM (AKA) authentication methods.
IETF working groups have designed three new protocols to develop standard solutions. Two of these protocols, PEAP and TTLS, originating from the pppext working group, are intended to solve the same problem as EAP/SIM and EAP/AKA, but independently of the specific EAP method. The idea in brief is to encapsulate the EAP protocol within TLS. The third protocol (PIC) is proposed to develop a unilateral version of the ISAKMP authentication and key derivation protocol, within which the EAP protocol is encapsulated. All three protocols include a method of managing secret parameters for further use. In PEAP and TTLS the keys are derived from the TLS master key, while in PIC the tunnel derived by simplified ISAKMP is used to transport security association from the local authentication server to the client.
By wrapping the EAP protocol within TLS, Protected EAP (PEAP) is said to provide user anonymity and built-in support for key exchange.
FIG. 1 shows the relationship between the EAP peer (client), the network authentication server (NAS) and the backend authentication server. The EAP conversation “passes through” the NAS on its way between the client and the backend authentication server. While the authentication conversation is between the EAP peer and the backend authentication server, the NAS and the backend authentication server need to have established trust for the conversation to proceed.
In PEAP, the conversation between the EAP peer and the backend server is encrypted and integrity protected within a TLS channel, and mutual authentication is required between the EAP peer and the backend server.
As a result, the NAS does not have knowledge of the TLS master secret derived between the EAP Peer and the backend authentication server, and cannot decrypt the PEAP conversation. However, in order to provide keying material for link-layer ciphersuites the NAS does obtain the master session keys, which are derived from the TLS master secret via a one-way function.
Since EAP methods may not know the link layer ciphersuite that has been negotiated, it may not be possible for them to provide link layer ciphersuite-specific keys. In addition, attempting to provide such keys is undesirable, since it would require the EAP method to be revised each time a new link layer ciphersuite is developed. As a result, PEAP derives master session keys, which can subsequently be truncated for use with a particular link layer ciphersuite. PEAP does not discuss the format of the attributes used to communicate the master session keys from the backend authentication server to the NAS. However, examples of such attributes are provided in RFC 2548.
The operation of PEAP is as follows:    1. Establish TLS connection. The TLS record protocol provides a secure connection between the peer and the back-end authentication server    2. Authenticate TLS server. The TLS handshake protocol is used for server authentication    3. Authenticate user. The user of the peer authenticates by tunnelling another EAP mechanism (e.g. Generic Token Card) inside the EAP-TLS connection. The back-end authentication server may have to contact another server to have the user authentication information validated.    4. Generate session keys. Using the TLS Pseudo-Random Function (PRF), the peer and the back-end server generate key material for use between the NAS and the peer.    5. (Transport session keys). The session key is transported from the server to the authenticator using, e.g., RADIUS attributes and a secure connection.
In the internet draft, EAP SIM GMM Authentication, dated August 2002, by Adrian Buckley et al., an application of PEAP to GSM authentication is presented. The architectural overview of this EAP method using SIM is depicted in FIG. 2.
As can be seen in FIG. 3, the architectural view of EAP-TTLS is essentially the same as in PEAP. EAP-TTLS claims to allow legacy password-based authentication protocols to be used against existing authentication databases, while protecting the security of these legacy protocols against eavesdropping, man-in-the-middle and other cryptographic attacks.
EAP-TTLS also allows the client and server to establish keying material for use in the data connection between the client and access point. The keying material is established implicitly between the client and server based on the TLS handshake.
When record layer security is instantiated at the end of a TLS handshake, a pseudo-random function (PRF) is used to expand the negotiated master secret, server random value and client random value into a sequence of octets that is used as keying material for the record layer.
EAP-TTLS leverages this technique to create keying material for use in the data connection between client and access point. The same PRF is used to generate as much keying material as required, with the constant string set to “ttls keying material”.
The PIC protocol is a method to bootstrap IPsec authentication via an “Authentication Server” (AS) and user authentication mechanisms (e.g., RADIUS). Referring to FIG. 4, the client machine communicates with the AS using a key exchange protocol where only the server is authenticated, and the derived keys are used to protect the user authentication. Once the user is authenticated, the client machine obtains credentials from the AS that can be later used to authenticate the client in a standard IKE exchange with an IPsec-enabled security gateway. The later stage does not require user intervention.
The PIC protocol realizes this approach and secures it using simplified ISAKMP and IKE mechanisms. The protocol embeds EAP messages (see RFC2284) in ISAKMP payloads to support multiple forms of user authentication. Once this user authentication succeeds, the client machine obtains from the AS credentials that can later be used by the client to perform regular IKE authentication with an IPsec-enabled gateway. PIC defines several forms of credentials and can be extended to support others. Note that this document uses the term “credentials” for both digital certificates and shared secret keys.
PIC requires no modification to IKE. Instead it uses simplified elements of ISAKMP and IKE to obtain a much less ambitious goal than general IKE, namely the secure provisioning of credentials for successfully authenticated users. The direct use of IKE, e.g., as compared to TLS tunneling, see GETCERT, reduces complexity and contributes to the efficiency of the protocol.
The PIC protocol is defined between the client and the AS. All other exchanges between the entities are implicit in the protocol. This applies in particular to user and machine authentication between the AS and the Back-End Authentication Server, and certification between the AS and the CA.
The four main stages of the proposed PIC protocol are:    1. An optional round of messages provides partial protection of the AS from denial-of-service attacks by verifying that the initiator of the exchange is reachable at the purported source IP address. This is done before any significant CPU or memory resources are consumed by the AS.    2. The protocol establishes a one-way authenticated channel from the client to the AS in which only the server is authenticated.    3. User authentication is performed over this secured channel. User authentication information is transported using EAP tunnelled within ISAKMP.    4. The AS sends the client a (typically short-term) credential, which can be used in subsequent IKE exchanges. This credential can be thought of as a certificate, or a private key generated or stored by the AS and accompanied by a corresponding certificate. It may also be a symmetric secret key, or other information for deriving such a key.
In stage 4 the created ISAKMP tunnel is used to obtain the secure provisioning of credentials for successfully authenticated users.
Another alternative proposal is HTTP digest authentication within a TLS tunnel, which is not described in detail herein.
It should be noted that the network entity referred to as the “back-end server” in the description of PEAP, see FIG. 1, does not have the same role in the authentication protocol as the back-end server in PIC. The PEAP back-end server is referred to as the EAP Server in FIG. 2, and its role is essentially the same as that of the TTLS AAA server of EAP-TTLS (FIG. 3) and the AS of PIC (FIG. 4).
The foregoing prior proposals all utilize an inner client (or mutual) authentication protocol within a secure tunnel that is constructed using a session key resulting from an outer server authentication protocol. In the following description EAP/AKA is used as an example of the inner protocol, however the general points of the discussion are applicable to any pair of authentication protocols used in the above manner. This is convenient since the EAP/AKA protocol is to be implemented by the USIM of 3G mobile stations, and thus this protocol is readily available to a terminal that is to be authenticated.
EAP/AKA is a challenge-response protocol. In the protocol the “responding entity” that is to be authenticated shares a secret key with the “challenging entity” that is to verify its authenticity. The challenging entity issues to the responding entity a challenge message which includes challenge data, which is typically randomly generated data. The responding entity then generates response data by applying a known function to that challenge data and to the secret key. The response data is returned to the challenging entity in a response message. The challenging entity can also apply the known function to the challenge data and the key, and the result of that can be compared to the response data. Only if the two match is the responding entity is authenticated. In the above described prior proposals the authentication method of AKA is to be used in just the same way as in its pre-existing applications: i.e. the terminal is supplied with challenge data to which it is to apply the function, and it then returns the result to the network for authentication. However, the key agreement part of UMTS AKA is not used.
In place of the EAP encapsulated AKA protocol, any EAP type protocol can be used for authentication of the terminal to the network.
FIG. 5 illustrates a logical architecture that is common to all of the above described prior authentication protocol proposals. First, a stage-one authentication exchange 40 takes place. In that exchange the identity of a back-end server 41 is authenticated to a terminal entity 42. However, the identity of the terminal entity is not authenticated to the back-end server. Then a stage-two authentication exchange 43 takes place, in which it is intended that the terminal entity is authenticated to the back-end server. Since each of the prior proposals is intended to be compatible with existing terminal entities, the existing authentication protocol (e.g. EAP/AKA), which has hitherto been usable as a stand-alone protocol, is used for the essential authentication part of stage two. In stage two the back-end server issues a challenge 44 to the terminal entity. The challenge is of such a type that the back-end server can (either by itself or through referral to a further authentication server) authenticate the terminal entity by means of its response to the challenge. The terminal entity processes the challenge and formulates its response (step 45) and returns that response to the back-end server (step 46). The back-end server checks the response (or refers it for checking) (step 47). If the response is correct then the back-end server has authenticated the terminal entity and can issue it with credentials, e.g., session keys that allow it to carry out subsequent secure transactions with the server (step 48). Otherwise no such credentials are issued and the terminal entity cannot carry out such transactions.
Some of the stage-two protocols have both an authentication function and a key-agreement function. In existing proposals the key-agreement function of the stage-two protocol is not used. Instead, the session keys are derived using the key-agreement function of the stage-one authentication protocol.
It was traditionally assumed that the above-described prior proposals provide for secure authentication of both the back-end server and the terminal entity to each other. However, it has been realized that this is not the case.
The vulnerability of the above-described prior proposals will be described with reference to FIG. 6. In FIG. 6 like items are numbered as in FIG. 5. In FIG. 6 a “man-in-the-middle” (MITM) attacker 49, who intends to impersonate a terminal entity 42, takes the place of the terminal entity in FIG. 5. The MITM attacker 49 performs the stage-one authentication with the back-end server. Then during the stage-two authentication, when the MITM attacker 49 receives the challenge message (step 44) from the back-end server it forwards it on to the terminal entity 42 (Step 50). Using the pre-existing AKA protocol, the terminal entity 42 returns the appropriate response to the MITM attacker 49 (Step 51). Then the MITM attacker 49 forwards that response to the back-end server (step 46). Since the response is the correct response for authentication of the terminal entity 42 the back-end server authenticates the MITM attacker 49 as the terminal entity 42 (step 47) and issues it credentials accordingly (step 48). The result is that this may allow the MITM attacker 49 access to supposedly secure services by impersonating the identity of terminal entity 42.
Reference can be made to Asokan, N., Niemi, V. and K. Nyberg, “Man-in-the-Middle in Tunnelled Authentication Protocols”, Cryptology ePrint Archive, The Cryptology ePrint Archive Report 2002/163, October 2002, for a detailed discussion of the MITM-type of attack.
The various proposals described above have been developed with the aim of using the existing challenge-response protocol for the essential authentication part of stage two, since the challenge-response protocol is already a feature of the terminals that are to use the system. However, and as was made evident above, these prior authentication protocols can fail to provide adequate security.
A problem of particular interest to this invention relates to the 3GPP IP Multimedia Subsystem, Release 5 and Release 6, functionality to authenticate User Equipment. The Release 5 and Release 6 services rely on the Digest AKA (IETF RFC 3310) for authentication. The Digest AKA specification defines a procedure to authenticate an endpoint using shared secrets and a cryptographic hash to produce credentials from a server-generated challenge. The password used in this process is based on the UMTS AKA cryptographic parameter RES.
However, in certain cases the Digest AKA is vulnerable to the tunneled MITM attack. In this attack, a malicious third party masquerades as the Digest AKA client over a secure transport channel to receive a challenge from the server. It then masquerades as a server to the actual Digest AKA client and forwards the received challenge to the actual Digest AKA client. The credentials received from the actual Digest AKA client are then sent to the actual Digest AKA server over the secure connection, allowing for the malicious third party to gain access to the resources of the masqueraded client.
In essence, the problem is based on the fact that the same set of credentials can be used in two different contexts, and that there is no way to distinguish between them.
Commonly assigned International Application PCT/IB02/05195 describes a method for authenticating a terminal in a communication system, where the terminal includes an identification means for applying authentication functions to input data to form response data, and the communication system is arranged to utilize a first authentication protocol for authentication of the terminal. The authentication functionality and the terminal share challenge data, and the terminal forms response data and a first key by applying the authentication functions to the challenge data using the identification means, and returns the response data to the authentication functionality. The authentication functionality authenticates the terminal by means of the response data and can apply an authentication function to the challenge data to duplicate the first key. The method includes executing a second authentication protocol wherein the terminal authenticates the identity of a network entity, and the terminal and the network entity share a second key for use in securing subsequent communications between the terminal and the network entity. A third authentication protocol is subsequently executed by: sharing challenge data between the network entity and the terminal; forming at the terminal a test data by at least applying one of the authentication functions to the challenge data by means of the identification means; transmitting a message comprising terminal authentication data, from the terminal to the network entity; and determining, based on the terminal authentication data, whether to provide the terminal with access to a service. In the determining step the terminal is provided with access to the service only if the terminal authentication data equals a predetermined function of at least the test data and the second key.
While the foregoing procedure is well suited for many applications, it would be desirable to provide an even more secure technique to thwart a MITM attacker, such as in the Digest AKA scenario.
It has been recently proposed to create a new version of Digest AKA, namely Digest AKAv2, Hypertext Transfer Protocol (HTTP) Digest Authentication Using Authentication and Key Agreement (AKA) Version-2, V. Torvinen and J. Arkko, sixteen pages, dated Jun. 16, 2003, also referred to herein as the AKAv2 draft). In this proposal a change is made to the way that the password in Digest AKA is generated, by including also the AKA sessions keys IK and CK in the Digest procedure as a “password”.
However, there are problems with this proposal, as will be discussed below.